This invention relates to lasers and other optical instruments having stringent boresight stability requirements and used in harsh environments; and more particularly, to improving boresight stability on an optical bench that uses a beam expander near an inlet or exit aperture of the optical system.
Optical instrument technology has evolved rapidly over the past forty years. For instance, early lasers had few components and consisted of a laser rod, flashlamp and cavity reflecting mirrors. As the industry matured and lasers became more capable, laser systems became more complex, adding components to support Q-switching, amplification, novel out-coupling schemes, polarization control, lateral and angular beam alignment, power monitoring, beam divergence control, improved mechanical and thermal stability, optical parametric amplification, and frequency doubling. As complexity grew, so did the number and variety of applications, and so did the need for improved boresight stabilityxe2x80x94maintaining the optical beam angle with respect to some reference.
Light Detection and Ranging (LIDAR) units, lasers, fire control systems, missile defense systems, interferometers, and other optical instruments rely on boresight stability to function properly. Lasers and other optical instruments require thermal and mechanical stability to maintain beam quality, output power, beam divergence and mechanical boresight. Lasers and other optical instruments used in precision applications such as surveying and targeting, and used in demanding operating environments, such as military applications, have very high stability requirements.
One of the primary causes of degraded optical system boresight stability is mechanical motion. The mechanical motion arises from a number of possible sources, including thermal effects and mechanical loads within the optical system, change in index of refraction over temperature (henceforth referred to as dN/dT) effects in the components, and motion of the bench itself due to external mechanical loads. Possible sources of movement include: mounting stresses, thermally induced stresses, material dimension instabilities, vibration, acceleration loads, and pressure changes, such as result from altitude changes. For example, as internal temperatures change, each of the materials in an optical system expands or contracts at a rate different than other materials in the system, which introduces movement of the various components with respect to each other.
Differential thermal expansions and contractions cause distortions in the optical bench, chassis, and motion of other component parts in the optical system. These movements can therefore alter the alignments of optical system parts with respect to each other, and in turn, cause an adverse change in the boresight alignment of the optical system. This adverse change causes the output beam to deviate laterally and angularly from its intended path, thus degrading optical system performance. Additional external environmental factors, such as changes in altitude or aircraft g-forces, also exert mechanical forces on system components that also can adversely impact boresight alignment by causing differential movement of optical system parts. This aggregation of design and environmental factors, and their resulting adverse effects on boresight alignment, can yield an unreliable optical system, especially for precision laser applications such as surveying, targeting, missile defense, long-range free space optical communications, and the alignment of machinery and buildings.
Laser energy is Gaussian in nature and subject to divergence. In most applications, collimated light energy is used to direct a laser beam to some specific location. In may seem counter-intuitive, but to form a bright narrow spot at some distance generally requires a larger diameter beam of light. A beam expander is an afocal telescope often used as the final output element on various laser instruments like range finders, designators, laser radar equipment, free space laser communications equipment and countermeasures systems. The beam expander, whether reflective or refractive, takes collimated input beam and outputs a collimated output beam of a larger beam.
The beam expander telescope is typically mounted proximate an exit aperture and on the optical bench with other optical components. Thus, the beam expander device is subject to the same factors that detrimentally affect the optical bench and boresight alignment of other optical system components such as mechanical motion due to thermal and mechanical loads within the optical system, and external mechanical loads.
However, the state of the art implementations have yet to satisfy the commercial applications and there is considerable room for improvement. Thus, there is a need for improving the boresight stability of optical systems that use a beam expander telescope.
The invention is devised in the light of the problems of the prior art described herein, accordingly, it is a general object of the present invention to provide a novel and useful apparatus and technique that can solve the problems described herein. The improved optical boresight stability system, as disclosed herein, meets the need identified hereinabove for improving the boresight stability of an optical system that uses a beam expander apparatus near an exit aperture and that operates in a variety of environmental conditions.
A beam expander on the output end of a laser or laser instrument is commonly thought to reduce the output boresight angular error inherent in the laser and/or instrument itself by a factor equal to the magnification ratio (MR) of the beam expander. This is a significant benefit, if actually achieved, as the optical elements can be mounted at lower costs as the tilt error will be reduced by 1/MR.
In order to obtain this commonly calculated and commonly expected benefit, the beam expander must be mounted in such a way that it is isolated from the motions experienced by the other optics. Since it is the last component in a train of optics, it can be mounted somewhat separately from the other optics so that movement of the optical bench does not affect the beam expander. In one embodiment the beam expander is located near the reference feature to which boresight will be measured. The beam expander, more than any other component, should be mounted in such a separated and rugged way that it moves negligibly with respect to that reference. This can be accomplished, for example, by placing the beam expander at the mounting feet or an external wall of the structure. If the motion relative to this reference surface is negligible in magnitude with respect to the system requirements, then and only then will the overall angular tilt error of the laser beam be reduced by the magnification ratio of the beam expander. The common standard for measuring the angular error is the reflective reference. Another option is to employ a specific sighting reference, but this adds to the cost and complexity.
Except in systems with very large magnifications and very loose angular stability requirements, this theoretical benefit is not experienced because of the interaction between the motion of the beam expander itself and the motion of the optics and the optical bed upon which the optics are mounted. This movement of the beam expander device diminishes any benefit to boresight stability. The problem is more pronounced when the beam expander tilts in the same direction as the optics or opposite to the optics. In order to obtain the theoretical optical advantages of reducing angular error, the beam expander must not move with respect to the output reference surface of the instrument, and this aspect has eluded designers for many years.
Boresight stability is improved by reducing the motion of the beam expander telescope with respect to other optical system components. The motion of the beam expander telescope relative to other components is minimized by several techniques that may be combined for optimal performance. The mounting location of the beam expander telescope should be away from thermal sources and preferably near a laser mounting surface. The beam expander telescope should preferably be mounted off the optical bench to provide isolation. The beam expander can be mounted on a rigid section of the chassis and preferably in the approximate reference feature of the laser. The beam expander can also employ a more rigid section for the section coupling the beam expander.
One embodiment to achieve a stable optical system with a beam expander is to use a separate optical bench with various optics onboard the bench and the input laser source either on the bench or piped in from a laser pump. The separate bench is mounted to the chassis, and mount the beam expander to the main chassis very near to the reference surface and in a way that minimizes motion with respect to the beam expander.
The beam expander telescope can also be mounted on a highly stiff portion for the integral optical bench and chassis. In some embodiments the present invention is a subassembly to a larger system and the beam expander can be mounted to a structure of the higher assembly. The stiff portion can be the same bench if manufactured with materials of sufficient strength. The stiff portion can also be reinforced using thicker dimensions of material or using the various supporting members that are well known in the art. The result is a laser or other optical system that has improved boresight stability due to its reduced sensitivity to beam angle errors, with such errors being determinable by a factor equal to the telescope magnification ratio, as further described in the Detailed Description hereinafter.
An object of the invention is an optical subassembly with boresight stability, comprising a chassis having a planar section and with an optical bench mounted to the planar section, wherein the optical bench has a plurality of optical elements mounted thereon. The optical elements have a common optical axis, and a beam expander device is rigidly mounted and isolated from the optical bench, and wherein the beam expander is coincident with the optical axis.
In addition, an object includes where the subassembly is mounted within a higher assembly and the beam expander is mounted to a supporting structure of the higher assembly.
Another object is the optical subassembly, further comprising a first rigid support section and a second rigid support section coupled perpendicular to the planar section, and wherein the beam expander is mounted to the first rigid support section. In addition, there can be supporting structures mounted to the first rigid support section, wherein the supporting structures are selected from the group comprising reinforcing plates and angular braces. Another feature includes where the first rigid support section is thicker than the second support section.
An object of the invention is the optical mounting assembly with boresight stability, comprising an integral chassis and optical bench, having a substantially planar section coupled between a first substantially perpendicular section and a second substantially perpendicular section. There are a plurality of optical components mounted to the planar section, wherein the optical components have an optical axis, and a beam expander isolated from the integral chassis is positioned coincident with the optical axis.
In addition, wherein the beam expander device is selected from the group comprising a beam expander telescope, a reflective device having a curved optical mirror with a central opening and a second mirror located substantially along an axis of the central opening, and an off-axis beam expander.
Yet another object is the optical mounting assembly wherein the beam expander is placed away from thermal sources.
An additional object includes wherein a boresight error is xcex82, an input beam tilt equals xcex81, and a beam expander telescope magnification ratio is MR, and wherein the boresight error is reduced according to the formula:
xcex82=xcex81/MR.
An object of the invention is an optical mount with improved boresight stability, comprising a laser source emitting a laser beam with an optical housing having a substantially planar section coupled between a first rigid perpendicular section and a second perpendicular section. There are a plurality of optical components mounted to the planar section, wherein the optical components have an optical axis, and wherein the laser beam is transmitted substantially along the optical axis. There is also a beam expander attached to the first rigid perpendicular section, wherein the beam expander is interposed along the optical axis and outputs the laser beam with a larger diameter, and wherein the beam expander is isolated from movements of the housing.
A final object is the optical mount with boresight stability, further comprising a reference feature, wherein the beam expander is proximate the reference feature.
Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention.